This invention relates to load pull and noise testing of microwave power and low noise transistors using automatic microwave tuners in order to synthesize reflection factors (or impedances) at the input and output of said transistors.
Modern design of high power microwave amplifiers, oscillators and other active components used in various communication systems, requires accurate knowledge of the active device's (microwave transistor's) characteristics. In such circuits, it is insufficient and inaccurate for the transistors operating in their highly non-linear or very low noise regions, to be described using analytical or numerical models only. Instead the devices must be characterized using specialized test setups under the actual operating conditions.
A popular method for testing and characterizing such microwave components (transistors) is “load pull” (for high power operation) and “source pull” (for low noise operation). Load pull or source pull are measurement techniques employing microwave tuners and other microwave test equipment. The microwave tuners are used in order to manipulate the microwave impedance conditions under which the Device Under Test (DUT, or transistor) is tested (FIG. 1).
There are essentially three types of tuners used in such test setups: a) Electro-mechanical slide screw tuners [1], (FIG. 1), b) Electronic tuners [2] and c) Active tuners [3], (FIG. 2).
Electro-mechanical tuners [1] have several advantages compared to electronic and active tuners, such as long-term stability, higher handling of microwave power, easier operation and lower cost. Electro-mechanical tuners use adjustable mechanical obstacles (probes or slugs)(1) inserted into the transmission media of the tuners (FIG. 3) in order to reflect part of the power coming out of the DUT and to create a “real” impedance presented to the DUT, instead of a “virtual” impedance created using active tuners in a setup as shown in FIG. 2.
Electro-mechanical tuners, as used in set-ups shown in FIG. 1, use the ‘slide screw’ principle, a tuning mechanism, as shown in FIG. 3; in this configuration the capacitive coupling between the vertical probe (1) and the central conductor (2) of the slofted airline (slabline)(3) creates a wideband reflection, Γ (or S11), of which the amplitude can be adjusted by modifying the distance “S” between the probe and the central conductor and therefore by changing the value of the capacitive between the central conductor and the probe.
In order to change the phase of the reflection factor S11 the RF probe (1), already inserted in the slabline (3), must be moved horizontally along the axis of the slabline and at constant distance from the center conductor (FIG. 3). This is accomplished using a lead-screw mechanism coupled with a stepper motor (FIG. 6); the lead screw (4) pushes a mobile carriage (5) along the slabline (6) axis; the carriage itself holds the RF probe (7) and can move it vertically in and out of the slabline.
The combination of both horizontal and vertical movement of the RF probe inside the slabline allows the creation of complex reflections factors S11 covering the entire Smith Chart (FIG. 8). Starting at point a, which corresponds to no reflection at all, we move the probe close to the central conductor thus creating a reflection and reach point b. Then we move the probe horizontally and turn on a circle of constant radius on the Smith Chart and reach point c.
There are two disadvantages to this approach: The first is that moving horizontally in order to change the phase of S11 takes a long time, especially at lower frequencies; the necessary horizontal travel, in order to cover 360° of phase, is lambda/2, where lambda is the electrical wavelength at a given frequency; at 1 GHz this is 15 cm, at 2 GHz 7.5 cm, etc. The second more important disadvantage, is that if the tuners are used in on-wafer setups, horizontal movement and tuner initialization create mechanical movements and vibrations of the tuners, which are transferred to the wafer probes (8) and may destroy the DUTs (=chips on-wafer) (9). FIG. 6 illustrates how the change of the center of gravity creates a “tilting” of the tuner and a movement of the wafer probes, which can be measured and is shown in FIG. 13.
Whereas horizontal RF probe movement is associated with movement of the massive mobile carriage (5), thus creating vibration problems, vertical movement (FIG. 7), even if it is also created using stepper motors, is associated with accelerating and decelerating much lower mechanical mass, and thus creates negligible or no mechanical vibrations (FIG. 15).
In order to determine the configuration necessary for this type of tuner to be able to tune over a considerable area of the Smith Chart using only vertical movement of the probes, a certain electrical distance between the probes L1 and L2 (FIG. 9) has to be chosen and maintained. The optimum configuration has been determined using an electrical equivalent circuit (model) (FIG. 11).
The model allows analyzing the microwave behavior of the circuit for the various horizontal and vertical positions of the probes and generates impedance plots on the Smith Chart (FIGS. 17–23).
It is the aim of this invention to propose a new tuner structure that employs, during normal measurement operation, only vertical movements, using three RF probes (slugs), inserted in the same type of slabline as tuners described in prior art.